CHEMISTRY COMMUNITY

Charisse Vu 1H

Galvanic and voltaic cells are the same.

Charisse Vu 1H

You would take take the standard potential of the cathode and subtract the standard potential of the anode from it.

Charisse Vu 1H

I don't think knowing the difference is too significant to this course and the final. I think it is just good to know that there are different kinds of catalysts.

Charisse Vu 1H

I felt that there was a typo, but maybe it was just Professor Lavelle's way of making sure we pay attention to the answers.

Charisse Vu 1H

The graphs only show order in relation to a specific molecule/reactant. I do not believe we know the graphs for overall orders.

Charisse Vu 1H

Why do we need to know how to calculate half lives? In what context would a half life question be given?

Charisse Vu 1H

Do we need to know how to derive the equation? I know that Professor Lavelle went over it in class, but I am not sure if we have to know how to derive it for the final.

Charisse Vu 1H

Why do we use the pre-equilbrium approach instead of the other methods presented in class?

Charisse Vu 1H

How are equilibrium constants and rate constants related? I understand there is an equation relating them, but I would like a conceptual explanation as to how they are related.

Charisse Vu 1H

How do you determine if a reaction is zero, first, or second order? Is there a way to tell without looking at a graph?

Charisse Vu 1H

Is there a difference between galvanic and voltaic cells? Or are they just two names given to the same thing?

Charisse Vu 1H

I think that conventionally, you always add things together rather than subtracting them. Zn is being broken down into 2 electrons and Zn2+. Although it does seem clearer to subtract the electrons because it lost them, I would just stick with adding 2 electrons to Zn2+ to keep things consistent.

Charisse Vu 1H

Electromotive force is simply the potential difference between two cells. Electron flow can go in any direction, depending on which side has the cathode/anode. But, it is typically calculated cathode-anode (right minus left).

Charisse Vu 1H

I believe that it only matters when H+ or OH- are involved in the reaction (when you are using the Nernst equation).

Charisse Vu 1H

Since it is a solvent, you don't need to put it in the cell diagram.

Charisse Vu 1H

Can someone summarize how le chatelier's principle and the nernst equation are related?

Charisse Vu 1H

Professor Lavelle says to do right minus left, because conventionally, the cathode is written on the right and the anode is written on the left. However, it should always be cathode-anode.

Charisse Vu 1H

I believe that you would assume that it is both an oxidizing and reducing agent. You should still split the reaction into 2 half reactions. Br2 --> Br- is the reduction reaction (0 to 1-) and Br2 --> BrO3- is the oxidation reaction (0 to 5+).

Charisse Vu 1H

Yes, Professor Lavelle said during lecture that when there are no conducting solids given in the problem (both the reactant and product are in solution), use Platinum as the electrode. He also said graphite works too but platinum is more common.

Charisse Vu 1H

But why is it that the maximum potential of the cell is when the switch is closed (very little current flowing)? What exactly does "maximum potential" mean?

Charisse Vu 1H

In the book, example 4I.3 explains this concept very well! I would go through that example and read some of the information in that section, as I believe the book explains delta S in more depth than what we covered in class.

Charisse Vu 1H

Do we have to memorize the oxidation number rules?

Charisse Vu 1H

Can someone go over how the van't hoff equation was derived from delta g and how K plays a role in the equation? Thanks!

Charisse Vu 1H

We always assume that delta S is the same at different temperatures because we can assume that the difference in delta S between the reactants and products is the same at two different temperatures.

Charisse Vu 1H

Do we include spectator ions in the equilibrium constant?

Charisse Vu 1H

When do we use Cv,m and Cp,m? Also, why and when do we use 5R/2, 3R/2, etc.? I am kind of confused on this, as I don't recall Professor Lavelle really explaining it.

Charisse Vu 1H

Steam at 100C causes a worse burn than water at 100C because steam at 100C has a lot more energy than boiling water at the same temperature. If you recall the phase change graph, a lot of energy is put into vaporizing water into a gas. During a phase change, the temperature remains constant, but the...

Charisse Vu 1H

4B.3: The internal energy of a system increased by 982 J when it was supplied with 492 J of energy as heat. (a) Was work done by or on the system? (b) How much work was done? I understand that work was done on the system for the internal energy to be increased by 982 J, but I don't understand why th...

Charisse Vu 1H

You can find a list of all the formulas and constants on the cover page of the first test we took. It has most of the formulas needed, but depending on the question, you may need to know how to manipulate equations to solve the question.

Charisse Vu 1H

Since the calorimeter contains the same volume of liquid in both cases (0.200 L and 100.0 mL+100.0 mL), there is no change in volume and the final volume is constant at 0.100 L. That is why the volumes are not considered since there is no change in volume.

Charisse Vu 1H

At constant volume, delta u = q because there is no work being done. Work requires that there is a change in volume, as the book defines work as "the process of achieving motion against an opposing force". At constant pressure, delta u = q - P*delta V because you can rearrange the equation...

Charisse Vu 1H

An ideal gas is a theoretical gas that does not have any intermolecular attractive forces, does not take up space, and obeys the ideal gas law.

Charisse Vu 1H

You can approximate x to be 0 when the Ka/Kb value is less than 10^-3. However, you also need to check that the x value you calculate is less than 5% of the initial amount.

Charisse Vu 1H

The bomb calorimeter is a constant volume calorimeter that is used to measure the heat of combustion of a reaction.

Charisse Vu 1H

A bomb calorimeter is a constant volume calorimeter. The reason why it is called a bomb calorimeter is because there is a large increase in pressure when a reaction occurs in the bomb vessel.

Charisse Vu 1H

I am a bit confused on which homework problems we should be doing for this week. It seems like the homework problems that we could do are all over the place (some in 4A, 4C, 4D). Are these the sections we should be doing problems from or am I missing something?

Charisse Vu 1H

I have picked up the final, but I am unsure if they will still be available this week because Professor Lavelle's email said that they would be available during week 3.

Charisse Vu 1H

How do we know which method to use when calculating the total enthalpy of a reaction?

Charisse Vu 1H

I believe using the standard reaction enthalpies would give you the most accurate answers. In the slides, it stated that if bond enthalpies are not available, then standard reaction enthalpies should be used.

Charisse Vu 1H

Hess's Law states that enthalpy change at each step of a reaction can be added together to give a total enthalpy change because enthalpy is a state function and is therefore additive.

Charisse Vu 1H

State properties are the same as state functions. The path that is taken to get from one point to another is not important. An example Professor Lavelle gave in class was altitude. Enthalpy is another example.

Charisse Vu 1H

Can the value of Kw ever change? I remember Professor Lavelle talking about temperature being able to change Kw but I don't really recall why or if temperature was the only factor that could change its value.

Charisse Vu 1H

I don't believe that lectures are bruincasted! They aren't found anywhere on his website.

Charisse Vu 1H

Kc and Kp both measure the equilibrium constant of a reaction. Kc is used when calculating in terms of concentration whereas Kp is used when calculating in terms of pressure. Typically, you would calculate the Kp of a reaction that is made up of all gases, but both Kp and Kc should be the same since...

Charisse Vu 1H

When you remove NO, the reaction favors the products because it wants to reach equilibrium. Since there is now less NO, and therefore products, the reaction proceeds forward to produce more products. This causes NH3 (reactant) to decrease.

Charisse Vu 1H

They will be available third week at 3034 Young Hall as per Professor Lavelle's email.

Charisse Vu 1H

Why is it that when pressure is increased by adding an inert gas to a reaction, the K value does not change? Also, what happens when pressure is decreased?

Charisse Vu 1H

Hi! When you make your ICE table, the x value that you use to add and subtract to and from the reactants/products is actually equal to .478 M, which was given in the original problem because it is given that the equilibrium concentration (the E section of the ICE table) is 0.478 M.

Charisse Vu 1H

You do take solids and liquids out of the equilibrium constant equation, but not including them does not eliminate the numerator. A quick google search indicated that the activity (I believe Professor Lavelle briefly mentioned that we would be considering concentrations to be the same as the activit...

Charisse Vu 1H

Remember that when solving a quadratic equation, x will have two values. The other value that you have not yet solved for will probably give you the right answer to the question! (Your quadratic equation is correct, you just need to solve for the other x).

Charisse Vu 1H

When pressure is increased by decreasing the volume, equilibrium shifts to to the side with less moles of gas because the concentrations of each compound is being increased according to the equation P=n/v. If pressure is increased by decreasing the volume, then the concentration increases since v is...

Charisse Vu 1H

I don't believe you need to know how to draw these compounds. I think you just need to know how to name/identify them.

Charisse Vu 1H

I think that this video is pretty helpful! https://www.youtube.com/watch?v=r4H5XjJPn58. Hope this helps :)

Charisse Vu 1H

Yes! Lyndon, one of the UAs, posted another review packet under the name "Marshmallow". There is also a chemistry review session this Friday!

Charisse Vu 1H

Why do certain ions in salts not affect the pH of the solution?

Charisse Vu 1H

The pH of pure water is 7. The equation to find pH is pH=-log[H3O+]. Substituting 7 into the equation, you would find that the concentration of hydronium ions is 1*10^-7. Then, to find the number of ions present, you would then do the following: \frac{1*10^-7mol}{L} (\frac{0.100L}{1})(\f...

Charisse Vu 1H

No, I don't believe you need to know the hybridization of terminal atoms. I believe you just need to know the hybridization of central atoms.

Charisse Vu 1H

I believe that the final exam will cover everything we learned this quarter. Just as how the midterm also covered the high school material that we covered in the first two weeks, the final will probably be similarly formatted and pull questions from the different topics we learned this quarter.

Charisse Vu 1H

I don't believe you would have to know how to draw out the lewis structures for acid/base reactions! However, I feel that knowing the lewis structures of acid/base reactions will strengthen and further your understanding of how these reactions work.

Charisse Vu 1H

Ka and Kb don't exist for strong acids and bases because they dissociate completely in solution. If you recall the formula for calculating the equilibrium constants, K_{a}=\frac{[H^{+}][A^{-}]}{[HA]} you will see that the concentration of the acid itself is in the denominator. Since strong acids/bas...

Charisse Vu 1H

Sigma bonds are actually stronger than pi bonds. Sigma bonds have more of a linear overlap whereas pi bonds have a "parallel" overlap. The electrons are held more tightly to the nuclei of sigma bonds than pi bonds. https://chem.libretexts.org/@api/deki/files/78352/CK12_Screenshot_9-20-3.pn...

Charisse Vu 1H

Yes! It would be perfectly fine to refer to intermolecular forces as stronger or weaker. It is true that the stronger the intermolecular force is, the harder it is to break the bonds between the molecules.

Charisse Vu 1H

It can be nonpolar if the dipole moments cancel each other out. For example, carbon dioxide has polar bonds but the overall molecule is nonpolar. The dipole moments face each other and the molecule is also symmetrical, cancelling each other out and making the molecule nonpolar.

Charisse Vu 1H

The electron geometry of H2O is tetrahedral because of the two sets of lone pairs. Therefore, its molecular geometry is bent (AX2E2).

Charisse Vu 1H

The molecular shape of carbon dioxide is bent because it has no lone pairs.

Charisse Vu 1H

Polar molecules tend to have higher boiling/melting points because they have greater intermoleculr forces than nonpolar molecules do. Nonpolar molecules only have van der Waals forces acting between molecules., which is the weakest form of intermolecular force. Polar molecules have dipoles that give...

Charisse Vu 1H

In a molecule, you can identify sigma and pi bonds by counting the number of single, double, and triole bonds it has. Every single bond is a sigma bond, every double bond has 1 sigma bond and 1 pi bond, and every triple bond has 1 sigma bond and 2 pi bonds.

Charisse Vu 1H

The OH group on the molecule is what makes the molecule polar. Its shape is a tetrahedral and the OH group is much more negative than the rest of the molecule.

Charisse Vu 1H

You would add an arrow. Make sure the bonds in the lewis structure are just lines!

Charisse Vu 1H

Sometimes, the angles can differ slightly because of how electronegative the atoms are in a molecule. If an atom is more electronegative, then the bond angle may be reduced.

Charisse Vu 1H

Hi!

You can write it however you want :) they are the same thing, but putting a subscript of 1 may make it clearer for the person grading the exams.

Charisse Vu 1H

Bent and angular are synonymous but typically, bent is used more often!

Charisse Vu 1H

The photoelectric effect states that there is a certain amount of energy (threshold energy) needed for an electron to be ejected. The frequency of the incoming photon is what determines whether an electron can be ejected. If the photon has a high enough frequency so that its energy surpasses that of...

Charisse Vu 1H

You can only use c=(wavelength)(frequency) and E=h(frequency) for light waves. De Broglie's equation is used only for things with mass, and light does not have mass. Therefore, De Broglie can not be used for light waves.

Charisse Vu 1H

Alpha represents the polarizability of an atom and depends on the number of electrons and size of the atom. R in the equation represents the radius. So, according to the equation, the interaction potential energy is proportional to $-\frac{\alpha _{1}\alpha _{2}}{r^{6}}$ .

Charisse Vu 1H

We don't necessarily need to know how to derive De Broglie's Equation. Just know that De Broglie's Equation is used when calculating the wavelength or frequency of anything with mass, such as electrons.

Charisse Vu 1H

Large distortion refers to the distortion of the electron cloud when an ionic compound is formed. The cation in the ionic compound pulls the anion's electrons closer to itself, distorting the electron cloud in the direction of the cation. That is why small cations have such high polarizing power.

Charisse Vu 1H

Polarizing power is the ability of an atom/ion to cause large distortions in the electron cloud of another, whereas polarizability refers to atoms or ions that readily undergo large distortions, such as iodide. Iodide is highly polarizable because it has a low effective nuclear charge and its outerm...

Charisse Vu 1H

Bond order is simply the number of bonds that link a specific pair of atoms (from the textbook). For example, the bond order in the molecule is three, since they are bound by a triple bond. To determine the most stable structure, you should try to get the formal charges on each atom of the molecule ...

Charisse Vu 1H

Hi! There's actually a really handy diagram that helps you determine electron configuration of an atom much easier. https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwi5gaHKrM_lAhUToZ4KHZjxBGwQjRx6BAgBEAQ&url=https%3A%2F%2Fwww.pinterest.com%2Fpin%2F326511041712732752%2F&...

Charisse Vu 1H

I don't believe that resonance structures are more reactive. In fact, I believe thay resonance actually stabilizes molecules because it delocalizes the molecule and lowers the overall energy. The electrons have a greater space to occupy.

Charisse Vu 1H

Ionization energy is the energy needed to remove an electron from an atom. It has a trend in the periodic table because as you move across a period from left to right, the ionization energy increases, and as you move up a group the ionization energy also increases. It increases from left to right be...

Charisse Vu 1H

Ionization energy refers to the energy needed to remove an electron from an atom (in its gas phase) while electron affinity refers to the energy released when an electron is added to an atom (in its gas phase). Electron affinity could also be explained as how much an atom attracts/gains electrons!

Charisse Vu 1H

The atom with the lowest ionization energy is commonly used as the central atom when drawing lewis structures! Dr. Lavelle talked about this briefly in lecture :)

Charisse Vu 1H

For b, there is actually only 1 orbital because n=4 refers to the 4th shell, l=2 refers to the d subshell, and m(l)=-2 specifies a single orbital. Therefore, part b only has 1 orbital. For part c, there are 4 orbitals because n=2 refers to the 2nd shell. In the second shell, the s and p subshells ex...

Charisse Vu 1H

I don't believe that we have to know how to find specific bond lengths. We just have to understand the differences in lengths of double and single bonds. Bond lengths within molecules with resonance must also be considered. The bond lengths in molecules with resonance are more like an "average&...

Charisse Vu 1H

Yes, your answer would be a summation of all the possible values for m and l given that n=2. If n=2, l must be 0 or 1 and m could therefore be -1, 0, or 1 if l=1. If l=0, then m could only be 0. The total number of orbitals then would be 4.

Charisse Vu 1H

When two electrons are paired, it means that they exist in the same orbital within a subshell. When electrons are parallel, they are filling the orbitals singly before pairing. According to Hund's Rule, this happens because when there are two or more orbitals with equal energies, electrons will fill...

Charisse Vu 1H

This question is dealing with deBroglie's equation, as it is asking for the wavelength of something that has mass and velocity (momentum). Since the units are not what we want, we must use dimensional analysis to convert ounces to kilograms and miles per hour to meters per second. It is given that o...

Charisse Vu 1H

You would use the DeBroglie equation when trying to determine the wavelength of anything with momentum (mass * velocity). It is used in particular for items with very small masses, such as electrons. Typically, when the question is asking about the wavelength or velocity of an electron or a small pa...

Charisse Vu 1H

The reason why Scandium is written as [Ar] 3d1 4s2 is because after the 20th element in the periodic table (Calcium), the 3d orbitals have a slightly lower energy than the 4s orbitals. Therefore, you would start filling the 3d orbitals until it is full. The 4s orbital is filled first because it is i...

Charisse Vu 1H

You would first need to find the empirical formula using the mass percentages. You would assume that there is 100g of the molecule and convert the grams of each atom into moles. Then, you would divide the moles by the smallest number of moles to find the ratio of atoms in the molecule. To find the m...

Charisse Vu 1H

I don't believe that sig figs are too important in this class. My TA told us that sig figs are not too big of a deal, as long as you do not round to whole numbers or to only 1 decimal place when the answer should clearly have 2 or 3 decimal places. As for scientific notation, I believe that it is mo...

Charisse Vu 1H

The 5 homework questions are always due in your discussion! As for the 5 posts for chemistry community, you need to have all your posts published by 11:59 PM Sunday night. I recommend that you contribute to discussions asking more conceptual questions, rather than homework problems. I feel that it i...

Charisse Vu 1H

I believe that rounding from 3.1 to 3 would be okay if you are solving for empirical formulas. If the number is about .1 away from a whole number, I don't believe that it would hurt to round it to a whole number. However, rounding to whole numbers in other problems besides solving for empirical/mole...

Charisse Vu 1H

This reaction is a combustion reaction, so O2 would be one of the reactants. Therefore, the chemical equation would be O2 + C7H16 -> CO2 + H2O. You would first balance the carbon, so you would put 7 as the stoichiometric coefficient in front of the carbon dioxide on the products side. Then, you woul...

Charisse Vu 1H

You just need to use dimensional analysis! Dimensional analysis utilizes the cancellation of units.

Charisse Vu 1H

Yes, I believe it's totally fine! It depends on the periodic table you are using too. Some periodic tables round off at different sig figs so you should be fine.

Charisse Vu 1H

I believe this question is simpler than what it seems. In the question, the gallium atoms are literally represented by the blue circles on the scale. If you count them, you would see that there are 9 "atoms" of gallium on the left hand side of the scale. Comparing the molar masses of galli...

Charisse Vu 1H

Since you are doubling the volume of the solution 90 times, the final volume would be much greater than the initial volume. The question asks how many molecules would be present in only 10. mL of the final solution. Since the volume has increased so dramatically, 10. mL of the final solution would c...

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